For a few reasons. I'll let some of the legal eagles on here weigh in but this is how I see it.
The NFL actually did violate the relocation guidelines in their own charter, and Amy Trask who was the CEO of the Raiders said as much in an interview and outlined in decent detail. Plus Demoff said some things after the fact that proved that he and Kroenke were deceitful and not negotiating in good faith. Goodell did the same thing while he was urging STL to continue their efforts while the entire time he already knew what Kroenke was going to do, and probably in some form gave it the green light. There may be servers with some proof of this that get seized who knows.
Plus we have some leaked info from the actual owners meeting itself when the vote was conducted when Jerruh said "you did it for the money" and that this was being done for the money.
There are also (I'm sure) PLENTY of emails between the city/county/etc and the NFL and other parties that would be damaging.
I am from CLE and when the Browns left they got sued and because of the lease were leveraged into leaving behind the name, logo, unis, hostory and all the rest.
Maybe this is a play for STL to get priority for a team that is relocating or a new team when expansion happens..........and as greedy as the NFL has become expansion is a given.
Just a side note.........anyone insulting the fans of STL for not supporting the team should take a close look in the mirror..........and a close look at the stands the last few games of this past season. Insulting a fan base is insulting all of us too. Poster were banned for that kind of thing in the past and I hope that continues. I don't give a freak who you are you don't have that right.
It's been a tumultous off season for the NFL, their image is in the dirt from the last few years. I cannot imagine a better time to attack and get them to the table to broker a deal.
For all we know they may just want the 20 or so MIL they spent trying to get things done while Kroenke and Demoff and Goodell were straight up lying to them.
Stay tuned as they say.